1. Field of the Invention
The invention concerns a process for producing plastic parts, in particular plastic molded articles such as, for example, plastic trays that are deep drawn in a shaping apparatus into molded articles from at two plastic foil strips.
2. Description of Prior Art
In one known process of this kind, plastic bulk articles such as trays for packaging meat, egg cartons, cups and plates made of a plastic film (foil, sheeting) e.g., of polystyrol, polyvinylchloride or polyethylene are produced in a compression molding press by vacuum or compressed air drawing, by means of special tools, and with the plastic film, heated beforehand on a strip heater. In this production phase, the deep-drawn plastic parts, referred to herein as "molded articles", are still joined together at their edges in a molded article unit. Detaching (separating) the molded articles is done by means of a punch press. Various other processing units, such as a printing unit, a hole station, a perforating mechanism or a depositing mechanism can be disposed between the compression molding press and the punch press, along with a stacking apparatus after the punching press. The transport apparatus for transport of the foil, of the molded article unit and of the molded articles from processing unit to processing unit generally consists of an intermittently driven, special chain conveyor gripping under both edges of the foil. During this transport movement, the molded article unit is, on the one hand, because of the cooling process, subjected to a change in size by shrinkage and, on the other hand, subjected to a displacement within each working cycle because of the summing of the play movements in the links of the individual chain members. The deviation of the operating position of the molded article unit from its set point position resulting within one work cycle because of this becomes all the greater, the greater the distance between two adjacent processing units.
Now, in order to have each processing procedure in the processing units, as well, in particular, as separating the molded articles in the punch press, occur at exactly the desired spot, an alignment procedure is needed for each processing unit. This alignment procedure is done based on the known process, either by the fact that the molded articles, because of their particularly conical form, center themselves in the associated tool or, when such is not the case, they center themselves by means of special centering cones co-formed in the foil strip when closing the particular processing tool.
This kind of aligning is particularly problematic in the case of very flat plastic parts with a very low sidewall cone or, in particular, if there is insufficient place available in the molded article strip for separate centering cones. In the case of the individual processing units, this unavoidably leads to irregularities and, when separating the molded articles at the punch press, even leads to non-equally running cut edges, in brief to a higher rejection rate, that in turn is reflected in the overall costs.
Therefore, proposed in German Offenlegungsschrift 28 52 697, has been a process through means of which the irregularities of the punched out edges of the molded article are avoided. This occurs by adjustment of the punch station relative to the forming station by means of an electric motor. Adjustment of the punch station is accomplished via a correction control circuit, that works with the aid of photocells that scan the hole marks punched into the molded articles in the forming station, and that correct the punch station in correspondingly positive or negative fashion in the running direction of the foil.
However, in spite of essential improvements, the accuracy of correction still displays the following deficiencies:
1. A 100 percent correspondance of the edges between form contour and punch contour is still not achieved.
2. The adjustment of the punch produced by the photocells for the purpose of correction requires a certain time loss, which negatively influences the capacity of the machine.
3. Upon conveyance from the forming station and insertion into the punching station, the foil cools. The molded articles are deformed by shrinkage and, therewith, even in the case of exact correction of the punch station, are punched out irregularly.
4. By cooling of the molded articles, essentially higher punching forces are required.
5. The overall length of the machine is too great because of the separate arrangement of forming station and punching station.
6. Energy consumption is relatively high, ocasioned by the use of auxiliary drives such as
(a) forming station drive, PA1 (b) hole station drive, PA1 (c) travel path drive for punch correction, and PA1 (d) higher forces of friction, and thereby-occasioned higher motor power in the foil feed.
7. Higher tool costs occur in the case of the known process because of use of separate forming and punching tools.
8. This also occasions considerable refurbishing times when changing the machine over to another article.
9. Expanded polystyrol has the property that, when expanding, formed are relatively compact external skins, while forming in the center, through means of the propellant, is a foam structure mixed with air bubbles.
Post-forming of the foil in the heating zone of a thermoforming machine causes expansion of the foil thickness from the initial thickness to a multiple, depending upon the temperature and the heating time. In so doing, the external skins remain almost compact. Now, when the molded article is formed and punched in separate stations in accordance with the known process, the foil cools down between the forming process and the punching process. The external skins are then no longer plastic because of the increased temperature, but rather brittle. During the punching process, the molded articles are separated based on the shearing cut principle. Therefore, remaining at the cut edges are thin, compact skins, and, therebetween, a foamed intermediate layer. Therefore, there is no homogeneity of the punched edges. The cut is raw and irregular. Reasons for this are the following: As already stated above, the foil, in the case of the separating process based on the known procedure, has already cooled and is solid. Through means of the shearing forces, all three structures (top external skin, intermediate layer, lower external skin) are separated by one same fast movement. In so doing, the three structures are detached separately from one another, one after the other. That is to say that the foam structure is open at the cut edges. Therefore, for example, water can infiltrate at this location. Additionally, these external edges are also exposed to destructive forces by mechanical influences.